One type of optical modulations has been knows as, what is called, quadrature amplitude modulation (QAM). A United States Patent, US 2009/244685A, has disclosed an optical modulator to modulate an optical signal by the QAM configuration. Another United States Patent, US 2008/232816A, has disclosed a transmitter module and a receiver module for an optical communication system with a polarization multiplexed configuration. Still another United States Patent, US 2012/148235A, has disclosed a control circuit for a transmitter module and a receiver module implemented within in the digital coherent system.
The digital coherent system has been known as a technique to enhance the transmission capacity. When an optical transceiver with the full-duplex function is implemented within the digital coherent system, various subjects are to be solved. That is, the coherent system not only requires an optical source to generate optical signals but inevitably requires another optical source, which is often called as a local source, in a receiver module. The requirement of two optical sources makes the optical transceiver with the full-duplex function hard to be formed in compact. For example, one standard relating to a housing, which is known as the “CFP2” standard, are quite hard to install two optical sources, an optical modulator, a coherent receiver, and so on within one housing.